1. Field of the Invention
This invention relates generally to photodetectors having surfaces sensitive to incident radiation. More particularly, this invention relates to photodetectors in which sensitivity of the photoconductive surface is increased by provision of a sensitizing surface layer.
2. Description of Related Art
The photoconductive effect, wherein incident radiation is absorbed by a material and free carriers generated when the incident photon energy raises surface electrons from one permissible energy level to another, is very commonly used in photodetectors sensitive to the intensity as well as wavelength of incident radiation. A variety of inorganic and organic photoconductive materials have been conventionally used in various applications depending upon their photoconductive characteristics. Commonly used photoconductive materials include inorganic materials such as Selenium, Cadmium Sulphide, Zinc Oxide and organic materials such as Trinitrofluorenone and selected members of the poly-N-vinyl group. The use of organic photoconductive materials has been restricted because of their slow response due to the inherently small electric charge carrier mobility. Although inorganic photoconductive materials exhibit higher mobility of the carriers for electric charge, it is practically difficult to control the sensitivity of such materials to the wavelength of incident radiation.
Several techniques have been used to extend and enhance the photoemission of a given material. These are based generally on charge transfer to the source electrode by means of a chemical bond, or the formation of a surface sensitizing layer which decreases the metal work function, or on enhancing the electromagnetic field of incident photons in the neighborhood of the conductive surface so that the increased photon-electron interaction produces a corresponding increase in photo-yield.
A popular and probably the most commonly used approach to increasing the photo-yield of a photoconductive material has been the application of a sensitizing surface layer, made of material such as cesium or sodium, on the conducting surface so that the interaction at the junction of the two layers lowers the photo-threshold. Cesium is most often used as material for the sensitizing layer and cesiated photocathodes, particularly those of the multi-alkali metal type, exhibit substantially higher response sensitivity than non-cesiated photocathodes. However, a number of disadvantages are inherent with the use of cesium for forming the sensitizing layer that make cesiated photocathodes inadequate for certain applications.
A major problem with cesium is its high reactivity to dust and impurities, as well as elements, such as oxygen and hydrogen, which commonly exist in the vacuum chamber environment where photocathodes are fabricated. This factor directly translates to a complicated fabrication process because of the need to maintain an impurity-free environment. In addition, the overall lifetime of the photodetecting surface is substantially reduced due to degradation of the photoconductive surface as a result of contact with dust, dirt, and other impurities. Further, there are theoretical and practical limits to the extent by which the sensitivity of a conducting material may be increased by the use of a cesiated sensitizing layer. Accordingly, there exists a need for a photoconductor displaying enhanced sensitivity and photo-yield in response to incident radiation, without being subject to the above disadvantages.